1. Field of the Invention
The present invention relates to a charge-coupled device (CCD) output processing stage and, more particularly, to a CCD output processing stage that amplifies signals from colored pixels based on the conversion efficiency of the colored pixels.
2. Description of the Related Art
Charge-coupled devices (CCDs) are semiconductor devices that are widely used in conventional image sensors to convert images of visible light into electronic signals that can be captured, transmitted, stored, and displayed. Examples of common consumer devices that utilize CCDs include camcorders and digital still cameras.
In operation, CCDs, which are formed on a layer of silicon, divide an image into a large number of discrete cells or pixels that are arranged in a number of rows (lines). In the silicon beneath each pixel, incident light is converted into a packet of electrons where the number of electrons in the packet represents the intensity of the incident light.
The electron packets formed within the pixels are then transferred across the CCD line by line to an output port. At the output port, the electron packets for each line are dumped, pixel by pixel, onto a diode, thereby forming a CCD output signal which has a number of pixel periods.
An analog processing stage senses the output signal, removes a black signal from the output signal, and outputs an amplified output signal. (The black signal, which varies over temperature and from part to part, represents the voltage that is present when no light is incident on a pixel.)
FIG. 1 shows a timing diagram that illustrates a conventional CCD image signal CCD OUT. As shown in FIG. 1, the CCD image signal CCD OUT has a number of pixel periods PP that include data pixel periods DP and black pixel periods BP. Each time the diode is reset, the image signal CCD OUT moves up to a reset level 10. Following reset, when no light is present, the signal falls to a black signal level 12.
On the other hand, when light is present, the signal level falls from the reset level to a pixel signal level 14. The intensity of the light incident on each pixel is then determined by removing the preceding reset signal level 10 and the black signal level 12 from the pixel signal level 14.
CCDs are often covered with color filter arrays (CFAs) which filter the light entering the CCD into a number of component colors. The pixels are covered by the CFAs so that each pixel receives only one of the colors. CFAs vary in the colors that are used, such as red, green, and blue, or cyan, magenta, yellow, and green, and the pattern in which the pixels that receive each color are arrayed.
When uniform white light shines on a CCD with an integrated CFA, the electron packets formed within the pixels should each have the same number of electrons since white light contains every color in equal proportion. In actual practice, however, the pixels associated with each color fill with differing numbers of electrons.
Thus, the pixels convert light from each of the colors into electrons at a different efficiency. The differences in the conversion efficiency of the pixels associated with each of the colors arises from transmission differences in the color filters as well as differences in the conversion efficiency of silicon across the frequency band of the incident light. As a result, a CCD image processing system must perform a function known as xe2x80x9cwhite balancingxe2x80x9d to equalize the signal levels of each color.
In digital imaging systems, an analog-to-digital (A/D) converter is used to convert the amplified signal level from the analog processing stage into a series of discrete digital numbers which represent the signal level of each pixel (after the preceding reset signal level 10 and the black signal level 12 have been removed).
In these systems, the analog processing stage is commonly implemented with a correlated double sampler (CDS) followed by a programmable gain amplifier (PGA) that matches the maximum signal level from the diode to the maximum input range of the A/D converter. If several colors are used in the system, it is general practice to match the color having the highest conversion efficiency (and thus the largest signal level) to the maximum input range of the A/D converter.
One problem with this approach is that the remaining colors, which have smaller maximum signal levels, do not use the full range of the A/D converter. Since, the pixels that receive the remaining colors have a smaller number of electrons, these pixels have smaller signal levels.
Since the noise levels of the CCD and the analog processing stage are fixed, the smaller signal levels of the remaining colors lead to a lower signal-to-noise (S/N) ratio for the remaining colors. The lower S/N ratio results in a picture with more noise in the remaining colors than in the color with the highest conversion efficiency.
In most systems, white balancing occurs after the A/D conversion, usually by applying a unique fixed gain to each remaining color in the digital domain. The problem with this approach, however, is that the poor signal-to-noise (S/N) ratios of the colors with the lower conversion efficiencies are set at the input of the A/D converter and, therefore, are not improved when multiplied by a fixed digital gain.
An approach to alleviate this problem is to use multiple PGAs in lieu of a single PGA so that each of the colors has a corresponding PGA. In this way, the maximum signal level of each remaining color can be amplified to be equal to the maximum input range of the A/D converter, and thereby obtain the best possible S/N ratio for each of the colors. Multiple PGAs, however, consume a large amount of power and are therefore not a preferred approach in battery or other low-power applications.
Thus, there is a need for an image processing stage which includes a single PGA that allows the maximum signal level of each color to be amplified to be equal to the-maximum input range of the A/D converter.
The present invention provides an image processing stage which allows the maximum signal level of each component color to be matched to the maximum input range of the A/D converter. The processing stage in the present invention processes an image signal that has data on a plurality of component colors.
The image signal also has a plurality of lines where each line has a plurality of black pixel periods and a plurality of data pixel periods. The black pixel periods have a plurality of black signal levels that are defined by a corresponding plurality of black pixels such that each black pixel period has a black signal level defined by a corresponding black pixel.
The data pixel periods have a plurality of data signal levels defined by a corresponding plurality of light-receiving pixels such that each data pixel period has a data signal level defined by a corresponding light-receiving pixel.
The image processing stage of the present invention includes a correlated double sampler (CDS) that is connectable to receive the image signal from a charge-coupled device. The CDS removes a reset signal level from both the black and data signal levels.
The stage also includes a digital-to-analog (D/A) converter that is connected to the CDS. The D/A converter receives a plurality of offset values. One of the offset values is received for the black pixel periods in a line, while the plurality of offset values are received for the data pixel periods in the line such that one offset value is received for each data pixel period in the line. Each offset value is converted into a corresponding offset signal and then output during a pixel period.
The stage further includes a programmable gain amplifier that is connected to the CDS and the D/A converter. The amplifier receives a plurality of digital gain values at a gain input such that a gain value is received for each pixel period. The amplifier amplifies the black and data signal levels in response to a gain value to form an amplified signal level in each black and data pixel period. The gain values represent the component colors such that each gain value represents a single component color.
The stage additionally includes an analog-to-digital (A/D) converter that is connected to the amplifier. The A/D converter digitizes the amplified signal levels in the black and data pixel periods to form a digitized black value for each black pixel period and a digitized data value for each data pixel period.
The stage also includes a controller that is connected to the D/A converter and the gain input of the amplifier. The controller stores and outputs the offset values to the D/A converter, and stores and outputs the gain values to the amplifier.
Further, in an alternate embodiment, an update circuit is also connected to the A/D converter and the controller. The update circuit determines an average digitized value from the digitized black values in a line, and compares the average digitized value with a digital reference value to determine a calculated value for the line. The calculated value of each line represents a component color. The calculated value for each line that represents the same component color is accumulated to form an accumulated calculated value. The controller then updates the offset value that represents the same component color as the accumulated calculated value in response to the accumulated calculated value for the line.
The present invention also includes a method for operating the processing stage. The method includes the step of removing a reset signal level from the black and data signal levels in the black and data pixel periods, respectively, in a line with a correlated double sampler to leave residual black and data signal levels in the black and data pixel periods, respectively, in the line.
The method also includes the step of removing an offset signal from the residual black and data signal levels in the black and data pixel periods, respectively, in the line with a digital-to-analog (D/A) converter to leave an offset-corrected signal level in the black and data pixel periods, respectively, in the line. The D/A converter, in turn, responds to an applied offset value.
The method further includes the step of amplifying the offset corrected signal level in each black and data pixel period in the line with an amplifier to leave an amplified signal level in each pixel period in the line. In addition, the amplified signal level in each pixel period is digitized so that a digitized black value is formed for each black pixel period and a digitized data value is formed for each data pixel period.
In accordance with the alternate embodiment, the method additionally includes the steps of forming an average digitized black value from the digitized black values in a line, comparing the average digitized black value with a reference value to form a calculated offset value, and accumulating the calculated offset value from each line that represents the same component color to form an accumulated calculated value. The applied offset value which the D/A converter responds to for the black pixel periods in the line is then updated in response to the accumulated calculated offset value.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized.